Design and physics basis for the upcoming DIII-D SAS-VW campaign to quantify tungsten leakage and transport in a new slot divertor geometry

Abstract

A set of experiments are planned to exploit the high SOL collisionality enabled by a tightly baffled slot divertor geometry to suppress tungsten leakage in DIII-D. A toroidal row of graphite tiles from the Small Angle Slot (SAS) divertor is being coated with 10-15 µm of tungsten. New spectroscopic viewing chords with in-vacuo optics will measure the W gross erosion source from the divertor surface with high spatial and temporal resolution. In parallel, the bottom of the SAS divertor is changed from a flat to a "V" shape. New SOLPS-ITER/DIVIMP simulations conducted with drifts using the planned "V" shape predict a substantial reduction in W sourcing and SOL accumulation in either B×∇B direction relative to either the old SAS divertor shape or the open, lower divertor. Dedicated studies are planned to carefully characterize the level of W sourcing, leakage, and scrape-off-layer (SOL) accumulation in DIII-D over a wide range of plasma scenarios. Various actuators will be assessed for their efficacy in further reducing high-Z impurity sources and leakage from the slot divertor geometry. This coupled code-experiment validation effort will be used to stress-test physics models and build confidence in extrapolations to advanced, high-Z divertor geometries for next-step devices.

Mitigation of plasma–wall interactions with low-Z powders in DIII-D high confinement plasmas

“…An essential difference in the present work is that wall conditions improve by injecting boron powder into the divertor (compared to an upstream injection location in the plasma crown). This suggests that divertor powder injection may also be beneficial for reducing high-Z impurity sources in a new tungsten divertor [53].…”

Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas

“…An important difference in the present work is that wall conditions also improve by injecting boron powder into the divertor (compared to an upstream injection location in the plasma crown). This suggests that divertor powder injection may also be beneficial for reducing high-Z impurity sources in a new tungsten divertor [45].…”

Predicting tungsten erosion and leakage properties for the new V-shaped small angle slot divertor in DIII-D

“…The C backgrounds were then used to estimate W erosion using a second iteration of DIVIMP, as described in section 5. Since DIVIMP models one impurity species at a time, the DIVIMP C simulations treat the W-coated regions as C-coated regions to approximate the effect of a steady-state carbon impurity fraction on the W surface based on previous experimental observations during the DIII-D Metal Rings Campaign [42] and time-dependent DIVIMP-WALLDYN erosion/deposition simulations for SAS-VW [18]. Predictions in [18] estimate that the equilibrium surface concentration of the W-coated region will be ∼80% W.…”

“…The sourcing and transport of W in SAS and SAS-V was characterized using the DIVIMP code with the C background plasma solutions presented in section 4. During the simulation, only the wall elements defined as 'W' in figure 2 were used as sources for W. Re-erosion of W from non-W surfaces is expected to be negligible due to the low surface concentration of W on those surfaces over the lifetime of the divertor, as predicted in [18]. Direct comparisons between the SAS shape and the SAS-V shape demonstrate the significant effect of the narrow V corner on W erosion and leakage from the divertor.…”

“…Informed by recent modeling, the DIII-D program will be modifying the current SAS divertor to create a new SAS-V divertor with a W coating on the outboard side of the slot (depicted in figure 1) [18]. The physics goals of this new 'SAS-VW' divertor campaign are to (1) characterize the high-Z leakage properties from a slot divertor and (2) assess the effect of a modified inner baffle on detachment in both B t directions.…”